Announcements

Please contribute 1-item to the Tondo Children Project

Dec. 17 or 19 Hour Exam 1 Coverage is Chapter 12 and 13.

Science uses three different ways to help us understand how solutes dissolve in solvents.

1. Practical Answer (the one that comes in handy in real-life) and related to #2.

2. Microscopic Answer (molecular approach using IMF’s--helps one understand the practical)

3. Macroscopic Answer (thermodynamic approach--cool if you like thermo---no need for molecules)

1. Practical Answer: “Likes dissolve likes”. This means:

• Non-polar molecules are soluble in non-polar solvents.

CCl4, gasoline, kerosene, hexane, oils, are soluble (miscible) in non-polar solvent benzene C6H6

• Polar molecules and ionic compounds are soluble in polar solvents

C2H5OH or NH3 or CH3COOH are soluble in H2O

2. Molecular Focus: The interplay of three kinds of IMF’s determines a solute’s solubility.

• weak solvent-solvent forces favors solubility• weak solute-solute forces favors solubility• strong solvent-solute interaction forces favor solubility

IonicSolid

Water molecules

Ion-Dipole

In this case, dissolution will only occur if ion-dipole forces (solvent forces) are stronger than ion-ion forces in the ionic solute.

The “likes dissolves likes” principles is explained by considering whether solute-solvent forces exceed solute-solute forces.

The “likes dissolves likes” principle can be applied to polar or non-polar systems. Recognizing whether the solute and solvent are polar or non-polar is key.

Non-polar solid (wax)weak London forces

Dissolution in a non-polar solvent

Strong Ion-Ion (or solute-solute force)

Strong London forces (or solvent-solvent force)

When solute-solute forces and solvent-solvent IMF’s are larger than solute-solvent forces dissolution will not occur. The phases will not be miscible.

Oil (non-polar) and water (polar) do not mix because solute-solvent forces far weaker than solute-solute and solvent solvent forces

Pentane, C5H12 and 1-butanol, C4H9OH have similar moleculer masses. Which of the two would be more soluble in water and why?

Pentane

1-Butanol

Predict which solvent will dissolve more of the given solute:

(a) Sodium chloride in methanol (CH3OH) or in propanol (CH3CH2CH2OH)

(b) Ethylene glycol (HOCH2CH2OH) in hexane (CH3CH2CH2CH2CH2CH3)

or in water.(c) Diethyl ether (CH3CH2OCH2CH3) in water or in

ethanol (CH3CH2OH)

Recognizing organic functional groups can help you recognize electronegativity differences, polarity and IMF’s for predictive power.

CH3 CH2 CH2 CH2 OHCH3 CH2 CH CH3

OHCH3 CH2 CH2 O CH3

CH3 CH CH2 OHCH3

CH3 COH

CH3CH3

CH3 CH2 O CH2 CH3

15-

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

CH3 CH2 CH2C OH

CH3 CH2 C CH3

O

CH2 CH CH2 CH3

O C

H2C CH CH3

OH

H

Figure 15.2 The chemical diversity of organic compounds

continuedH2C

H2C CH

O

CH3

O

H2C C CH3CH3

CH3 CH C

O

CH3

H

15-

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Table 15.1 Numerical Roots for Carbon Chains and Branches

Number of C atomsRoots

1

2

3

4

5

6

8

7

9

10

meth-

eth-

prop-

but-

hex-

pent-

hept-

oct-

non-

dec-

PREFIX + ROOT + SUFFIX

Quiz 2

Please put your books away

Quiz 2

1. Which of the following pure liquids would likely have the highest viscosity at a particular temperature and why?

A. C2H5OC2H5

B. CH3CH2ClC. CH3COOCH3

D. OHCH2CH2OH

2. State the single main IMF that opposes vaporization in each of the following substances.

hexane (CH3CH2CH2CH2CH2CH3)watermethyl amine (CH3NH2)

(c) Ethanol - Diethyl ether can interact through a dipole and dispersion forces. Ethanol can provide both while water would like to H bond.

(b) Water - Hexane has no dipoles and is non-polar. It will not interact with the -OH groups in ethylene glycol. Water can H bond to the ethylene glycol.

SOLUTION:

Predict which solvent will dissolve more of the given solute:

(a) Sodium chloride in methanol (CH3OH) or in propanol (CH3CH2CH2OH)

(b) Ethylene glycol (HOCH2CH2OH) in hexane (CH3CH2CH2CH2CH2CH3) or in water.(c) Diethyl ether (CH3CH2OCH2CH3) in water or in ethanol (CH3CH2OH)

PLAN: Consider the intermolecular forces which can exist between solute molecules and consider whether the solvent can provide such interactions and thereby substitute.

(a) Methanol - NaCl is ionic and will form ion-dipoles with the -OH groups of both methanol and propanol. Propanol is less hydrophillic (water loving and polar) to the dispersion forces to a greater extent.

SOLUTION:

Predict whether each of the following substances is more likely to dissolve in carbon tetrachloride or in water.

(a) heptane C7H16 (b) sodium sulfate

(c) HCl (g)

Carbon tetrachloride is a non-polar hydrocarbon solvent while water is a polar molecular solvent. We use the “like dissolve likes” principle. Heptane is a non-polar non-ionic hydrocarbon with only dispersion forces as is I2. Sodium sulfate is ionic and highly polar as is HCl. Water is better suited for the latter while carbon tet for the former.

(d) I2

Arrange the following substances in order of increasing solubility in water (least soluble to most).

(a) CH3CH2CH2CH2CH3

(b) OH-CH2CH2CH2CH2CH2OH (c) CH3CH2CH2CH2CH2OH (d) CH3CH2CH2CH2CH2Cl

a < d < c < b

3. Macroscopic Approach: Thermodynamics helps us explain the energetics of dissolution of a solute in a solvent. The Gibb’s Free Energy is a thermodynamic state function that determines whether a process (dissolution) will occur spontaneously or not.

ΔG < 0 reaction happens spontaneouslyΔG > 0 reaction won’t happenΔG = 0 equilibrium

There are 3-cases:

ΔG = ΔH - T ΔSTemperature

Gibb’s Free Energy Change

Change in Entropy

Change in Enthalpy

Entropy is another thermodynamic state function that can be thought of (for now) as one of nature’s driving forces that strives to maximize statistical randomness, or disorder in all systems.

Some Enthalpies and Entropies of Solution in Water at 25˚C.

Which of the following would be the best ice packs and hot packs?

CaCl2

KBr

NaBrMgSO4

NaClLiCl

KCl

NH4NO3

-91.217.2

3.9-37.0

-0.6

19.9-57.7+25.7

Thermodynamics is used to explain the energetics of dissolution of a solute in a solvent.

We can consider the dissolutionprocess as the sum of three steps.

1. Separate solute molecules2. Separate solvent molecules3. Form solute-solvent molecules

ΔH1: Separate solute molecules

ΔH2: Separate solvent molecules

ΔH3: Form solute-solvent